This invention relates to structures comprising an element of polyethylene bonded to a surface of a metal substrate and to methods of making such structures. More particularly, but not exclusively, the invention relates to structures in the form of a laminate of polyethylene film and metal in the form of sheet or foil which may be formed into an article.
British Patent Specification No. 1,295,132 describes a method of forming a laminate of polyethylene and various metal substrates. In one example given, a low density polyethylene film was pretreated by corona discharge and then applied to preheated tinplate by means of a rubber coated roller. The laminate was allowed to cool whereon the peel strength of the bond between the pretreated polyethylene and the tinplate was 33 lb. per linear inch (150N/linear inch). When the method was repeated using an untreated polyethylene the coating could readily be stripped off the tinplate.
Unexpectedly we have found that a particular range of the now available polyethylenes called "linear low density polyethylenes" can be bonded directly to various metal substrates by the influence of heat and pressure. The term "linear low density polyethylene" is becoming widely used in the plastics industry. (See for example "Modern Plastics International" March 1980 p. 35; European Plastics News" March 1980 p. 8; and "Modern Plastics International" 1979 p. 26, p. 76).
In this specification the term "linear low density polyethylene" is used to denote a group of co-polymeric materials of low to medium density, produced by low pressure co-polymerisation of ethylene and an alpha olefin co-monomer, using an appropriate catalyst.
The molecular arrangement is believed to consist of a regular linear main-chain backbone with a number of short side chains. Where the co-monomer is but-1-ene, the side chains are formed by ethyl groups. The polymers show different behaviour from conventional low density polyethylenes made by the high pressure process, in particular by having a higher melt temperature.
The invention provides a method of making a laminated structure including an element of polyethylene bonded to a surface of the metal substrate by engaging the polyethylene element with the surface of the metal substrate and bonding it directly thereto by the application of heat and pressure, characterised by the step of selecting the polyethylene to be a linear low density polyethylene having a density from 0.91 to 0.94 g.ml.sup.-1 and a melt flow index in the range 0.2 to 2.0 g/10 min, produced by low pressure co-polymerisation of ethylene and but-1-ene.
The metal substrate may be chosen from various metals such as steel; steel having a coating of tin or chromium or chromium/chromium oxide or zinc; aluminium; aluminium alloy; nickel; zinc or copper. The metal substrate may, if desired, have a conversion coating thereon such as the passivation coating on tinplate or oxide on aluminium.
Preferably the polyethylene element is the form of a film and the metal substrate is in the form of a sheet or foil. If desired the polyethylene film may be engaged with one side of the sheet metal or to opposed sides of the sheet metal to make a laminate of polyethylene/metal/polyethylene. Conversely sheet metal may be engaged with opposed surfaces of the polyethylene film to make a laminate of metal/polyethylene/metal. If desired the polyethylene may contain fillers or pigments such as titanium dioxide.
In a first embodiment of the method the polyethylene and metal substrate are heated together to a temperature within the range 200.degree. C. to 300.degree. C.; sufficient pressure is applied to exclude any risk of bubbles between the polyethylene and substrate. The pressure is preferably less than 3500 kPa, a particularly preferred range of pressure being between 200 kPa and 2400 kPa. The pressure is preferably applied for a period in the range 0.1 seconds to 5 minutes, the shorter times being applicable by pinch rolls and the longer times being applicable by press tools.
Preheat may be applied to the polyethylene or to the metal substrate or to both the polyethylene and the metal substrate.
If desired the polyethylene may be pretreated by a pretreatment chosen from corona discharge, glow discharge or flame treatment; however this pretreatment is not essential. The metal substrate may be flame treated before engagement with the polyethylene.
In a second embodiment of the method the polyethylene and metal substrate may be bonded to a relatively low strength at a temperature in the range 140.degree. C. to 300.degree. C., after which the structure is heated without applied pressure at a temperature in the range 180.degree. C. to 300.degree. C. to create a stronger bond. Preferably this period of heat without pressure does not exceed 5 minutes.
The structure produced by the method may be formed into articles by folding to a pouch or tray or alternatively drawing between a punch and die to make a hollow article. After such mechanical working operations it may be desirable to heat the article in order to ensure that the polyethylene is bonded to the substrate.
We have found that linear low density polyethylenes of the type defined above made by the low pressure process, will provide a means to achieve high bond strength between the polyethylene and metal substrate without recourse to adhesives, with less energy input than is required for conventional polyethylenes.